1. Field of the Invention
The present invention relates to a stacked liquid crystal display system for forming an image, for example, a color image, by superposing images displayed on a plurality of liquid crystal display cells.
2. Description of the Related Art
There have previously been proposed a variety of liquid crystal display systems for forming an image by superposing images displayed on a plurality of liquid crystal display cells. One such system is constructed by stacking component members one on top of another, in the order of a first polarizer, a first STN (supertwisted nematic) liquid crystal cell, a second polarizer, a second STN liquid crystal cell, a third polarizer, a third STN liquid crystal cell, and a fourth polarizer (U.S.P. Pat. No. 4,917,465, Japanese Patent Examined Publication No. 59-38565).
This liquid crystal display system is capable of providing a color display by combining the retardation values of the liquid crystal display cells with associated color polarizers. To produce colors, the system uses a substractive color mixture process by which three primary colors, cyan, yellow, and magenta, are mixed together to produce eight colors, white, cyan, magenta, blue, yellow, green, red, and black. It is also possible to produce a greater number of colors, such as 16, 64, and 128 colors, by displaying the three primary colors in various luminance and by varying their light transmittance ratios.
In the above stacked crystal display system, the three liquid crystal display parts, each comprising an STN liquid crystal cell and associated color polarizers disposed adjacent thereto, function to produce the three primary colors, cyan, yellow, and magenta, respectively. That is, a liquid crystal display part for producing cyan, a liquid crystal display part for producing yellow, and a liquid crystal display part for producing magenta are stacked one on top of another to construct the display system.
As described above, in the so-called stacked liquid crystal display system for producing a display by combining a plurality of liquid crystal display cells, the liquid crystal layers of adjacent liquid crystal display cells are separated by glass substrates, as components of the liquid crystal display cells, and a polarizer, thereby increasing the spacing between the liquid crystal layers, i.e. the spacing between the display surfaces. The resulting problem is that an apparent separation occurs between the superposed images as the angle between the observer's line of sight and the normal to the display surface area increases (the angle is hereinafter referred to as the viewing angle).
Therefore, in such stacked liquid crystal display systems, transmitted light is extracted in parallel, for example, by an overhead projector (OHP), so that the image formed on the liquid crystal display is projected in enlarged form onto a screen in front. In such a projection system, apparent separation of images due to an increase in the viewing angle can be avoided.
However, as it requires a high intensity backlight (light source) for projection of light, the power consumption increases, rendering battery operation impracticable. Furthermore, the projection system has such problems that it requires the provision of accessory optics, such as a Fresnel lens for screen projection, making the system further complex in construction and larger in size, and also that the projection for enlarged viewing lowers the image definition of the display.
These problems defeat the advantages of the liquid crystal display that is low power, can be driven by batteries, and is thin and light weight in construction, and thus limit the range of potential applications for the stacked liquid crystal display system.